The present invention relates to a turbine casting having cast-in cooling bumps along a surface to provide improved heat transfer between a cooling medium and the opposite side of the casting. Particularly, the present invention relates to methods for salvaging castings with defective cast cooling bumps by applying a coating to areas of the defectively cast bumps to improve their heat transfer characteristics.
Various techniques have been devised to maintain the temperature of gas turbine components below critical levels. For example, a cooling medium such as coolant air from the turbine compressor or steam is often directed to the component along one or more component surfaces. Such flow is understood in the art as backside flow, where the cooling medium is directed at a surface of the component not directly exposed to high temperature gases of combustion. Enhanced heat transfer is also accomplished by providing cast cooling bumps along the backside flow surface. For example, cast cooling bumps may be provided in a gas turbine on the inside surfaces of the stage 1 and stage 2 nozzles. It will be appreciated that the outer surfaces of the nozzles are exposed to the hot gases and are subject to very high temperatures along the hot gas path exposed side thereof. A cooling medium such as steam or air flows through various cavities within the nozzles along the interior nozzle surfaces to provide backside cooling flow. The cast-in bumps on the interior surfaces of the nozzle have a generally hill-like shape and are spaced from one another to provide a coolant side surface area larger than that of the baseline smooth surface area.
In certain gas turbine components, for example, nozzles and shrouds, the cast-in cooling bumps are sometimes defective. By defective cast bumps is meant that one or more bumps are missing from the surface of the cast part or the bump is only partially formed. These defects occur as a result of manufacturing process limitations. When the parts are cast and inspected, defective areas can be identified and the parts are sometimes scrapped. This results in a significant financial loss. Accordingly, there is a need to provide a method for salvaging cast parts of a turbine that have defective cast cooling bumps.
In accordance with a preferred embodiment of the present invention, there is provided methods of salvaging a casting having cast cooling bumps projecting from a surface thereof wherein one or more areas of that surface have defectively cast bumps manifested by one or more missing or partially cast bumps. To accomplish the foregoing, the surface area or areas manifested by one or more missing or partially cast bumps are first identified by visual inspection or thermography. Once identified, the area or areas are cleaned and the defective bumps removed, e.g., by grinding or grit blasting. Thus, partially formed bumps may be ground down to the surface area between the bumps, or the smooth area or areas with partially formed bumps may simply be roughened. After cleaning, cooling enhancement material is applied to the surface area(s) manifesting defectively cast bumps and the spaces between the defectively cast bumps. Preferably, a coating containing particles, e.g., metal particles, is applied to the defective area. For example, a metallic powder is set in intimate contact with the defective area and brazed thereto by an electric arc wire sprayed thermal process. The size of the metallic powder particles is selected to provide heat transfer enhancement on the local defective surface area. The alloy of the metallic powder particles is chosen to withstand the part operating conditions while providing a high thermal conductivity. The alloy must also withstand the part operating temperature while not diminishing other part properties, i.e., LCF. By applying the coating in the local area of the defectively cast bumps, the part can be salvaged and utilized, notwithstanding the lack of bumps or partial bump formation in one or more areas of the backside surface. It will be appreciated that this salvage or repair technique can be used on most or all of the gas turbine parts having cast bumps, such as shrouds, certain stator nozzles, buckets and the like, for enhancing heat transfer.
In a preferred embodiment according to the present invention, there is provided a method of salvaging a casting having cast cooling bumps projecting from a surface thereof wherein at least one area of the surface has defectively cast bumps manifested by one or more missing or partially cast bumps, comprising the steps of identifying the defectively cast area and thermally spraying a coating on the defectively cast area to form an overlying coated surface having a coated surface area in excess of the uncoated defective surface area to afford enhanced heat transfer across the casting relative to the heat transfer across the casting without applying the coating.